Polaritonic Materials: A Comparative Study on the Relaxation Kinetics in Fabry–Perot versus Plasmonic Cavities

ABSTRACT Strong exciton–photon coupling results in the formation of hybrid light–matter states whose relaxation dynamics often experience a bottleneck due to a large manifold of dark states. This bottleneck results in dynamics like those of the uncoupled material, hampering the observation and utilization of polaritonic properties. A possible solution to this problem is to use cavities with fewer dark states, suggesting a need for an increased understanding of the influence of the cavity platform on the polariton photophysics. Here, we use pump‐probe spectroscopy to compare Fabry–Perot cavities and plasmonic nanorods coupled under identical conditions and analyze how the cavity influences relaxation kinetics. Fabry–Perot cavities host a large number of dark states, leading to dynamics fully dominated by the bottleneck. However, the relaxation from polaritonic states in nanorod systems is distinctly different compared to the uncoupled materials, with a sub‐100 fs decay that dominates the kinetics. This fast relaxation emerges only when selectively exciting within the polaritonic region: off‐resonance excitation yields plasmonic hot‐carrier dynamics. These findings show that the cavity critically defines polariton relaxation. Moreover, this work suggests that plasmonic cavities can operate as either polaritons or plasmons, offering tunable functionality for applications in, for instance, polaritonic chemistry.